Abstract: The characteristic method was used in simulating computation of nonsteady gas flow process in the exhaust system of model 15A petrol engine． Some boundary conditions were discussed and improving suggestions were proposed. The computed results are in good agreement with the measured values. It shows this calculating method and computer program can be used for designing of exhaust system of small twostroke petrol engine． Furthermore, this program can be used for analyzing the effect of parameters on exhaust pressure wave and predicting engine performances.